Flexographic printing has become the major printing process to produce flexible packages for food and non food products, especially in North and South Americas and share roughly an equal part of the production of gravures in Europe.
In Asia and in the Middle East the share of flexographic participation is still increasing since its quality continues to grow and the capability to print Asiatic characters is now easily obtained.
Flexographic printing has achieved many improvements since its invention like the anilox rolls that bring more consistency in the inking process, and the introduction of the closed inking chamber, that reduces exposure to volatile solvents present in the ink and maintains the ink viscosity stable for longer periods. Photopolymers were certainly one of the biggest contributions to quality, followed by direct laser engraving in the last 10 years. All of those contributions has forced the development of better inks and one of the most important components of those better inks is their color strength.
There is a close similarity between printing quality and screening resolution, anilox screening and volume of ink, particularly ink color strength. To improve the printing quality it is mandatory to increase also the screening that we are using. Gravure and offset use 150 to 200 lines per inch while the traditional flexography screening requests ranges between 100 and 140 lines per inch. The ability to avoid the smallest dots in the plate to penetrate into anilox cells defines the relation between the screening of plates and the anilox screening since for the anilox this relation is about 6-8 times bigger than for the plates.
In order to print 200 lines per inch it is necessary an anilox with 1200 to 1600 lines per inch, and as the anilox lines increase, the volume of inks to be transported decreases fast, even using the new YAG laser engraving technology to bring higher volume to the anilox rolls, demands of stronger inks to achieve the specified color densities for printing still existing.
Table 1 below shows a standard anilox charter available for flexographic printing nowadays.
TABLE 1Anilox Screening versus VolumeL/inch150205250305355410460510560610660710Min.19.07.05.54.53.83.22.92.72.42.21.71.6Max.118.014.011.59.57.26.25.35.04.23.83.24.4L/inch760815865915965102011201220132014001500Min.11.51.51.41.41.41.31.21.21.21.01.0Max.14.24.04.03.53.02.92.62.42.01.81.61Volume in BCM (billions of cubic micron per square inch)
The reduced volume of high anilox screening as shown above is one of the great limitations on traditional flexographic inks which contain 50% to 70% of solvent in their composition reducing the possibility to increase the pigment load in the ink and consequently the ink color strength.
To increase the complexity in order to achieve high color densities demanded by the flexographic printing process it is not easy to transpose to the substrate all the ink present in the anilox since the ink layer remains partially on the anilox roll and in the printing plate surfaces.
The high Volatile Organic Compounds (VOC) and the low density color strength are two main residual problems for flexographic printing, constituting the next challenge to be achieved: obtaining a better quality ink and also developing a friendly environmental ink for the flexographic printing process.
U.S. Pat. No. 5,690,028, relates to a viscous radiation curable ink and the decrease of the ink viscosity by heating it before the application. After applied, the ink layer cools down and the viscosity increases again to an amount enough to support the overprint of other color and give a satisfactory color trapping. The main disadvantage verified on U.S. Pat. No. 5,690,028 is the difficulty to control the temperature of the ink and ensure that no significant variation occurs during the printing process.
Other inventions tried to solve these problems in many different ways. In the U.S. Pat. No. 6,772,683, incorporated herein by reference, it is suggested to use low viscosity flexographic printing inks which have a viscosity controlling diluents to implement wet trapping of the sequentially applied ink layers by controlling time between the ink layer. However, the time required for the solvent evaporation is too long.
U.S. Pat. No. 7,479,511 discloses a water based formulation using basically the same concepts of above mentioned U.S. Pat. No. 6,772,683 in terms of how the ink layers can be overprinted, but also focuses on the mobility of the reactive materials inside the final applied film, since lack of molecules mobility can lead to a low degree of conversion after the cure process.
In addition, U.S. Pat. No. 7,479,511 also uses some water retention to guarantee the necessary mobility of the system in order to achieve the desirable conversion degree. A correct amount of water is proposed as a compromise between minimum retention level and ability to withstand the overprint process in flexographic printing.
PCT/US2005/012603 proposes a layered material having two or more layers that can be curable by exposure to highly accelerated particles such as an electron beam. The layered material comprises a substrate, an ink formulation on at least a portion of the substrate. The ink formulation comprises ink and a monomer curable by free radical or cationic polymerization, and a lacquer comprising at least one monomer curable by free radical or cationic polymerization.
The above discussed solutions require high investments to add ultraviolet radiation (UV) or Electron Bean radiation (EB) installations and even the cost of the inks are high in comparison to the traditional solvent inks. Those patents are based exactly in the same principle that occurs in the traditional solvent based system, since the intermediate drying in the Central Drum Flexographic machines is not strong enough to result in a completely dried ink layer.
Evidence that interstation drying of the flexographic printing process is not capable to assure complete drying of the ink, is given by a continuous research for low tack resins for flexographic solvent inks, since the tack of the non completely dried ink creates problems to the printing process, including color trapping and plate dust among others.
On the other side, the increased viscosity of the above discussed inks results in the difficulty to reach a total conversion of all reactive materials due to a low mobility created by the high viscosity, problem that U.S. Pat. No. 7,479,511 tries to solve by leaving some amount of water up to the moment that the ink passes through the curing system (EB or UV) and by implementing a complex balance of the presence of water.